Due to continuing improvements in materials technology, modern aerospace vehicles include an increasing amount of structural components made of composite materials. Because vehicle components made of non-conducting composite materials may become degraded when subjected to electrical discharge (e.g. lightening strike, electromagnetic effects (EME), etc.), such components are typically coated with an electrically conductive material, such as conductive paints, anti-static coatings, thermal sprayed coatings, and the like.
Throughout various stages of development of such aerospace vehicles, measurements are often made of electrical resistance of a conductive layer that is disposed on a composite component of the vehicle. One known test device that has been successfully used for this purpose is shown in FIG. 1. As shown in FIG. 1, the prior art test device 100 includes first and second conductive strips 102, 104 disposed on a non-conductive layer 106 that is attached to a non-conductive substrate 110. In this example, the substrate 110 includes a flexible, compliant layer 111. Each conductive strip 102, 104 is operatively coupled to a conductive lead 112, 114 that extends from the test device 100 to a suitable piece of test equipment 120, such as a digital ohmmeter.
As further shown in FIG. 1, the conductive strips 102, 104 pass through the non-conductive layer 106 to an inner side of the non-conductive layer 106 (shown in phantom) prior to passing around an end 113 of the substrate 110. On the end 113, first and second auxiliary contact members 115, 116 are disposed on the non-conductive layer 106. Each of the first and second auxiliary contact members 115, 116 is electrically coupled to a corresponding one of the first and second conductive strips 102, 104, respectively, by a plated-through hole 117.
In operation, the test device 100 may be used by pressing the first and second conductive strips 102, 104 into engagement with a conductive layer 122 (not shown) to be tested. The test equipment 120 then measures the electrical resistance RT of the conductive layer 122 between the first and second conductive strips 102, 104 in ohms per square. Because the first and second conductive strips 102, 104 are disposed on the compliant layer 111, the non-conductive layer 106 and conductive strips 102, 104 may flex to conform to the curvature of the conductive layer 122. In an alternate mode of operation, the first and second auxiliary contact members 115, 116 may be pressed into engagement with the conductive layer 122 under test, and the resistance RT of the conductive layer 122 is then determined by the test equipment 120. Due to their relatively smaller size, the auxiliary contact members 115, 116 may be used on smaller surfaces in comparison with the first and second conductive strips 106, 107.
Although desirable results have been achieved using the prior art test device 100, recent developments in conductive coatings are placing increased demands on such apparatus. For example, in the past, conductive coatings have been characterized by relatively high resistance per square values which were readily capable of accurate measurement using the prior art test device 100. More modem conductive coatings, however, have relatively smaller resistance per square, thereby posing a greater challenge to such test apparatus.
As the resistance of the conductive coating 122 decreases, the additional component of measured resistance attributable to the contact resistance between the surfaces of each conductive strip 102, 104 and the conductive coating 122 becomes an ever-increasing percentage of the resistance measured by the test equipment 120, thereby increasing the uncertainty associated with the measurement. In some cases, the resistance of the conductive coating 122 may even be smaller than the component of contact resistance between the conductive strips 102, 104 and the conductive coating 122, thereby preventing accurate measurement of the resistance of the conductive coating 122 using the prior art test device 100. The contact resistance may also fluctuate depending on the force applied by the user to the test device 100 during testing, thereby introducing an additional component of uncertainty between successive test measurements. Therefore, there is an unmet need in the art for an improved test device capable of accurately and consistently measuring the resistance of modern, low resistance conductive coatings.